Equipment & Gear, Exclusives, Patient Care

The Role of Point-of-Care Ultrasound (POCUS) in Prehospital Cardiac Arrest

With cardiac arrest being one of the most common critical conditions treated by EMS providers, the apparent improvement in patient centered outcomes yielded by POCUS would certainly justify more widespread use of ultrasound in the prehospital setting, writes Mathew Kent. (Photo/Adam Mason)

In recent years, Point-of-Care Ultrasound (POCUS) has proved to be useful in the prehospital setting for a wide range of clinical conditions including trauma, stroke and cardiac arrest.1–5 However, only approximately 4% of ambulance services in the United States and Canada currently have ultrasound on their units.6 The discrepancy between the potential uses for POCUS and its current prevalence in emergency medical services (EMS) is likely due to high cost combined with a lack of evidence suggesting that POCUS is correlated to an improvement in patient centered outcomes.

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However, recent evidence suggests that the use of POCUS in the management of cardiac arrest alters treatment protocols and may improve neurologically intact survival, although this has not been studied directly.7,8 Additionally, the uses of POCUS for cardiac arrest described by these studies are feasible in the prehospital setting with minimal additional training.5,9–11 With cardiac arrest being one of the most common critical conditions treated by EMS providers, the apparent improvement in patient centered outcomes yielded by POCUS would certainly justify more widespread use of ultrasound in the prehospital setting.12,13

Utility of POCUS in the Termination of Resuscitation

Currently, the majority of EMS services using POCUS in their cardiac arrest protocols use it to aid in the decision to terminate resuscitative efforts by identifying the presence or absence of cardiac activity. This is achieved using a subxiphoid view and can be accomplished quickly, keeping pauses in chest compressions to under 10 seconds.12 Several studies have shown that brief education programs can adequately prepare paramedics without prior POCUS training to perform this assessment.5,11

According to one study, the identification of the absence of cardiac activity using POCUS in the prehospital setting has a 97.5% positive predictive value for death prior to leaving the ED.14 This shows that POCUS has great potential to guide decision making regarding prehospital termination of resuscitation, but should be used in conjunction with other assessment modalities.

POCUS Pulse Checks

ALCS guidelines recommend that pulse checks take less than 10 seconds to perform.15 In fact, evidence suggests that for every five second increase in the pulse check time, mortality increases by 18%.16 However, one study found that the median time needed to perform a manual pulse check for EMS providers was 24 seconds. Furthermore, 45% of the patients in the study were not identified as having a carotid pulse despite having a blood pressure above 80 mmHg systolic.17 An alternative approach to manual pulse check is the POCUS pulse check.7 In this approach, the ultrasound probe is placed on either the carotid or femoral artery and the ultrasonographer checks for both collapsibility of the vessel (arteries should not be easily compressible) and visible pulsation of the vessel. Compared to manual pulse checks, POCUS pulse checks can be performed in the same amount of time—in less than five seconds and can be performed by providers who are inexperienced in ultrasonography.

Additionally, POCUS pulse checks have a higher first attempt success rate of identifying a pulse than manual pulse checks during cardiac arrest.7 Furthermore, POCUS is able to detect pulses that do not generate enough pressure to be felt by manual palpation and as a result can differentiate between pulseless electrical activity (PEA) and pseudo-PEA which otherwise could not be identified in the prehospital setting.8,12,18 This distinction significantly changes how we approach the treatment of cardiac arrest and the return of spontaneous circulation (ROSC). It is therefore essential that EMS services more widely employ POCUS in order to deliver the highest possible level of care.

POCUS in Identification of Reversible Causes of Cardiac Arrest

Asystole and PEA are associated with several reversible causes. Previous studies have shown that PEA with a reversible cause (pericardial effusion) has a higher survival rate to discharge (15.4%) than PEA without a reversible cause (1.3%).19 If identified properly, these causes can be promptly reversed in the prehospital setting or transport can be initiated to the ED for definitive treatment.12,19 Historically, the identification of the reversible causes of PEA and asystole has been guided by the five H’s and T’s pneumonic outlined in the ACLS guidelines.15 However, the recognition of these reversible causes has been largely limited to gathering a sound history. Recently, evidence has emerged suggesting that POCUS may be useful in the identification of these reversible causes in the prehospital setting.1,12,20

Helman et al. propose that POCUS assessment during cardiac arrest can identify profound hypovolemia, tension pneumothorax, cardiac tamponade, pulmonary embolism and pleural effusion, without prolonging pauses in chest compressions.8 These reversible causes can be systematically ruled out utilizing the Rapid Ultrasound for Shock and Hypotension (RUSH) exam.21 This is a quick and simple exam and its components have proven to be adequately performed by paramedics.5,10,11,22 One study examining the utility of POCUS in prehospital cardiac arrest, found that the findings on the focused echocardiography during life support (FEEL) exam changed the treatment trajectory or destination selection in 89% of patients when done during cardiopulmonary resuscitation (CPR) and in 66% of cases when done during the peri-arrest period.23 Although POCUS is extremely useful in identifying the mechanical reversible causes of PEA and asystolic arrest, information in the patient’s history should be used to assess for the metabolic, toxicologic and hypoxic causes of cardiac arrest.

Reversible Causes of Cardiac Arrest Identified via Ultrasound

Pseudo-PEA: A Diagnosis Made Possible by POCUS

The terms PEA and pseudo-PEA are becoming increasingly common in the resuscitation literature.8,12 Pseudo-PEA is not technically cardiac arrest, but is instead a profound shock state that in the era before POCUS could only be identified through the placement of an arterial line. Pseudo-PEA, unlike PEA, has been associated with a ROSC rate with good neurologic outcome of 50%.24 In pseudo-PEA, there is cardiac activity with a perfusion capable rhythm that produces a pulse, but the pulse is only strong enough to be detected by a POCUS pulse check, not by manual palpation.25 This is an important distinction as standard cardiac arrest treatments may cause harm in pseudo-PEA.8,26

Additionally, evidence suggests that pseudo-PEA may represent between 42% to 86% of PEA cardiac arrests, furthering exemplifying the importance of the detection of this condition in the prehospital setting.25 In cardiac arrest, epinephrine (1mg of 0.1mg/ml) has been associated with worse neurologic outcomes in some studies.27,28 The reasons for this observation may be related to the total dosage or timing of administration, but this is still under intense debate in the field of resuscitation.28 Some experts argue however, that the deleterious effects of the cardiac arrest dosing of epinephrine could be exponentially higher in cases of pseudo-PEA, highlighting the importance of the identification of the condition in the prehospital setting.8,26

It is important to note that this has not been directly studied to our knowledge. Several alternative strategies to standard cardiac arrest dosing of epinephrine have been proposed, including prompt administration of 5-20 mcg of push dose epinephrine (0.01mg/ml) or 20U of vasopressin.8,24,26 These initial treatments should serve as a bridge to a norepinephrine infusion at a dose of 10-50 mcg.8 The theory behind these treatments is that the vasoconstriction provided by a background infusion of norepinephrine will help transform contractility producing a weak barely palpable pulse, to a contractility capable of producing a perfusion capable blood pressure. The utility of chest compressions in pseudo-PEA is debated as some animal studies suggest that chest compressions during cardiac contractility may impair preload and subsequent cardiac output.29

However, newer evidence shows improvements in cardiac output in animal models when synchronized mechanical compressions are performed with ongoing cardiac contractility.30 Further research is needed, but in the meantime clinical judgment should be used in the decision to withhold chest compressions. Some experts recommend using a mean arterial pressure (MAP) of 50 mmHg as a threshold for discontinuing chest compressions.26 In theory, this is logical as most studies put 50-55 mmHg as the lowest MAP capable of perfusing the brain.31 Additionally, other markers of cerebral perfusion can be used including a persistent pulse oximetry waveform and end tidal CO2 after cessation of chest compressions.32,33

These methods may be especially useful markers in pseudo-PEA as blood pressures below 80 mmHg are difficult to obtain and multiple attempts at obtaining a blood pressure may delay clinical decision making.12 In fact, in the presence of a good pulse oximetry waveform, a pulse oximetry blood pressure can be obtained to gauge a ballpark systolic blood pressure. This is performed by inflating the blood pressure cuff until the waveform disappears and then slowly deflating the cuff, observing where the waveform returns as the systolic blood pressure.34

A New Approach to ROSC

ROSC is generally assessed for by looking for changes in end tidal CO2 or by assessing for the presence of a palpable pulse or obtainable blood pressure. However, in cardiac arrest management utilizing POCUS, there are several in-betweens. Weingart et al. suggest that ROSC should be looked as a four-part spectrum.26 First, there must be evidence of a perfusion capable rhythm. Then, there must be cardiac contractility followed by a pulse (either palpable or identified by POCUS). Finally, that pulse must be strong enough to generate an obtainable blood pressure. As described before, patients in the middle ground with a POCUS pulse, but not a palpable pulse or measurable blood pressure require a modified treatment plan.

For example, the traditional approach to ROSC might miss a pulse that is too weak to be palpated as well as miss the associated blood pressure as it is too low to be assessed by non-invasive means. A POCUS pulse would identify the weak pulse which would prompt vasopressor titration to achieve an adequate MAP (>65 mmHg) and prevent the continuation of ACLS care that has the potential to cause harm in those with pseudo-PEA. In the event of a POCUS pulse with an unobtainable blood pressure, push dose vasopressors and a norepinephrine infusion should be started empirically and other markers such as pulse oximetry and end tidal CO2 (as stated previously) should guide decision making when determining to resume or pause chest compressions.

A Modified Cardiac Arrest Protocol Utilizing POCUS

In the new era of POCUS, the traditional protocol for cardiac arrest management should be revisited. However, the need for high quality chest compressions with minimal interruptions as well as prompt defibrillation of shockable rhythms is still at the forefront of optimal care. The differences are instead in the details and are outlined below.

Upon arriving at the scene of a cardiac arrest, initiate standard care per ACLS guidelines.15 The initial pulse check is likely to be performed by manual palpation as providers should not waste time setting up POCUS equipment to perform the initial pulse check as this will delay time to starting compressions and defibrillation if clinically indicated. During the first round of compressions the ultrasound should be prepared. Just prior to the first rhythm/pulse check, the ultrasound should be placed on the femoral or carotid artery. During the pause in compressions, one provider should analyze the rhythm while the provider using the ultrasound assesses for the presence or absence of a femoral or carotid pulse.

This check should be binary (yes/no) and should not take more than five seconds. If a POCUS pulse is detected, it should be followed up by assessing for a palpable pulse. If there is both a POCUS pulse and a palpable pulse, standard ROSC care should be initiated per local protocol. If there is a POCUS pulse, but no palpable pulse, then the modified pseudo-PEA algorithm should be followed (See figure 2).

Figure 2

If there is no POCUS pulse, standard ALCS care should continue to the next pulse/rhythm check. Prior to the second pause in compressions, a curvilinear ultrasound probe should be positioned in the subxiphoid position without effecting the quality of chest compressions. While compressions are still underway, the ultrasonographer should attempt to gain an adequate view of the heart. If resources allow, a second ultrasound should be positioned on the femoral or carotid artery. If there is only one ultrasound available, the second pulse check should be done manually.

When compressions are paused, one provider should again analyze the rhythm while the provider with the ultrasound records a clip of the heart. This clip should be less than five seconds to keep the pause in compressions as short as possible. This clip can then be interpreted after compressions resume and should be evaluated for cardiac tamponade, pulmonary embolism (right heart strain) and cardiac activity including the presence of fine ventricular fibrillation that may have not been apparent on the ECG. If a POCUS pulse is now detected, move to the modified pseudo-PEA algorithm or standard ROSC care based on presence of a palpable pulse.

If there is no POCUS pulse, resume ACLS care and use information from ultrasound to treat underlaying causes or initiate transport (using clinical judgement) to a destination that can treat the underlaying cause. Other reversible causes of cardiac arrest can now be evaluated for using the RUSH exam. These assessments can be done while compressions are ongoing and should not interrupt chest compressions or interfere with their quality. If reversible causes are found, they should be managed per local protocols or clinical judgement should be used to decide if initiation of transport to the ED would be beneficial. POCUS should be used for the remainder of the arrest for pulse checks and in the event of the detection of a POCUS pulse without a palpable pulse, the modified pseudo-PEA algorithm should be initiated.

Following this step-wise approach to cardiac arrest management, we can better understand the physiologic etiology of the patient’s cardiac arrest and can therefore employ individualized treatment. POCUS may also shorten the duration of pulse checks as it can be accomplished in under five seconds.7 However, it is important to note that in some studies POCUS was associated with longer pulse check times, likely due to inadequate training and poor resuscitation communication.35 If POCUS is used after proper training and communication, it has the potential to transform the field of prehospital resuscitation as we continue to learn about how to best utilize it in the setting of cardiac arrest and the peri-arrest state.

References

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